«Thy glass will show thee how thy beauties wear,
Thy dial how thy precious minutes waste».
The youth’s aging face will be reflected in a mirror, and the passage of time will be reflected on his watch, clashing with the youth’s eternally young thoughts.
Thy glass will show thee how thy beauties wear,
As the young man ages, each wrinkle on his face will remind him of a memory from his youth. However, because the young man will not be able to remember every event from his life, the poet gives him a notebook in which to write his thoughts. This notebook, like the poet’s sonnets, will become a history — a record of the young man’s beauty while he was yet young.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com